The present invention relates to a coating solution for preventing adhesion of polymer scale and a method for preventing adhesion of polymer scale to a polymerizer during polymerization of a monomer having an ethylenically unsaturated double bond, for instance, a vinyl ester or acrylic acid.
There has been known a problem that, when a polymer is prepared by polymerizing a monomer in a polymerizer, the polymer adheres to the inner wall of the polymerizer in the form of scale. The adhesion of such scale results in the reduction of cooling capacity of the polymerizer and yield of intended polymers. Moreover, if the scale peeled off during the polymerization is mixed in the product, the resulting polymer becomes nonuniform and the quality thereof is hence impaired. For this reason, it is necessary to remove scale every two or three batches. This requires a great deal of labor and time. Furthermore, the scale contains poisonous unreacted monomers and thus sufficient care would be necessary for operations.
In the polymerization of a monomer having an ethylenically unsaturated double bond, there have been known methods for preventing scale-adhesion which comprises, for instance, applying a variety of substances onto inner walls of polymerizers as scale-inhibiting agents. Examples of known substances suitable for use as scale-inhibiting agents are dyes and pigments such as those disclosed in Japanese Patent Application Publication Nos. 45-30835 and 52-24943; aromatic amine compounds such as those disclosed in Japanese Patent Provisional Publication No. 51-50887; and reaction products of phenolic compounds with aromatic aldehydes as disclosed in Japanese Patent Provisional Publication No. 55-54317. The scale-inhibiting methods which make use of these substances as scale-inhibiting agents are effective in polymerization of halogenated vinyl monomers such as vinyl chloride or mixtures comprising halogenated vinyl monomers, as principal components, and a small amount of other monomers copolymerizable therewith.
However, most of the conventional scale-inhibiting agents represented by those disclosed in the foregoing patents are colored and hence the colored scale-inhibiting agents applied onto the inner walls of polymerizers are mixed in a polymerization system through dissolution or peeling off of the colored scale. This leads to coloring of the resulting polymer product and a reduction of quality thereof.
In the foregoing scale adhesion-preventing methods, the scale adhesion-inhibitory effect of the foregoing scale-inhibiting agents varies depending on a variety of conditions such as the kinds of monomers and polymerization initiators (catalysts) used; manner of polymerization; and materials for the inner walls of polymerizers. Thus, it has been very difficult to effectively inhibit scale adhesion with certainty.
Further, if a polymerization initiator having a strong oxidative effect is used, such as potassium peroxodisulfate, ammonium peroxodisulfate, acetyl cyclohexylsulfonyl peroxide or bis(2-ethoxyethyl)peroxy dicarbonate, a scale adhesion-inhibiting agent in the form of a coating is decomposed through oxidation and this results in the reduction of the scale adhesion-inhibitory effect. Moreover, polymer scale is easily adhered to the inner walls of polymerizers made of stainless steel or other steels as compared with the inner walls of polymerizers provided with glass lining.
If monomers to be polymerized are those represented by the following general formula: EQU CH.sub.2.dbd.CR.sup.1 R.sup.2
(wherein R.sup.1 represents a hydrogen atom or a methyl group and R.sup.2 represents a hydrogen atom or a group --C.sub.n H.sub.2n+1, --COOH, --COOM in which M is an alkali metal or ammonium ion, --COOC.sub.n H.sub.2n+1, --CN, --C.sub.6 H.sub.5, --C.sub.6 H.sub.4 R.sup.2 in which R.sup.3 represents a hydrogen atom, --OH, --CH.sub.3 or --CH.dbd.CH.sub.2 ; --OCOC.sub.n H.sub.2n+1, --OC.sub.n H.sub.2n+1 or --CH.dbd.CH.sub.2, and nis an integer), such as styrene, .alpha.-methylstyrene, acrylates and acrylonitrile, these monomers have high abilities of solubilizing the coating film formed according to the foregoing scale adhesion-inhibiting methods. For this reason, a part or whole of the coating film is dissolved away and thus effective inhibition of scale adhesion cannot be ensured. Among these, styrene, .alpha.-methylstyrene, acrylates and acrylonitrile have extremely high capacities of solubilizing the coating films of scale adhesion-inhibiting agents and thus, any desired scale adhesion-inhibitory effect cannot be obtained. The scale adhesion is a severe problem when a polymerizer of stainless steel is used.